Conventional ion traps, of the kind described in U.S. Pat. No. 2,939,952, are generally composed of three electrodes, namely a ring electrode, and a pair of end caps, with appropriate RF and DC voltages applied to these electrodes to establish a three-dimensional field which traps ions within a mass range of interest in the relatively small volume between the ring electrode and the end caps. The electrodes may be hyperbolic, producing a theoretically perfect three-dimensional quadrupole field, or they may deviate from hyperbolic geometry, giving rise to additional multipole fields superimposed on the quadrupole field and which can produce improved results.
Usually ion trap mass spectrometers are filled in an essentially mass-independent manner and are emptied mass-dependently by manipulating the RF and DC voltages applied to one or more of the electrodes. The ion storage and fast scanning capabilities of the ion trap are advantageous in analytical mass spectrometry. High analysis efficiency, compared to typical beam-type mass spectrometers, can be achieved if the time to eject and detect ions from the trap is smaller than the time required to fill a trap. If this condition is met, then very few ions are wasted.
However an inherent disadvantage of ion traps is that ion transport into the trap is usually of very low efficiency, e.g. one to ten percent, primarily due to the relatively small volume of the trap and the very demanding ion energetic constraints for trap acceptance of externally generated ions. The relatively small volume of the ion trap means that the number of ions that can be accepted before space charge effects become serious is also relatively small. Increasing the radial dimension of the volume of the trapping chamber of a conventional ion trap partially overcomes this limitation, but with the additional disadvantages of reduced analytical utility and/or increased costs (e.g. reduced mass range, larger power supplies). The small volume of the ion trapping chamber will also tend to limit the linear response range (i.e. dynamic range), again because of the effects of space charge at high ion densities.
An additional problem is that when a conventional ion trap is performing an analysis, no additional ions can be accepted. For many modern ion sources such as electrospray, ion spray (disclosed in U.S. Pat. No. 4,861,988), or corona discharge, this can be a considerable disadvantage because the trap fill time is usually short compared with the analysis time. Consequently, and as described in U.S. Pat. No. 5,179,278 assigned to the assignee of the present invention, many ions can be wasted during the analysis time, resulting in relatively low duty cycles.
It is known that ions can be trapped and stored very efficiently in a two-dimensional RF quadrupole. In some cases ions have been admitted into and then trapped in a two-dimensional quadrupole for purposes of releasing them into a conventional ion trap, as shown in U.S. Pat. No. 5,179,278. More generally ions have been admitted into a pressurized linear cell or a two-dimensional RF quadrupole for the purpose of studying ion molecule reactions. Generally the ions enter the device from a mass selective source such as a resolving quadrupole, are trapped for a specified period of time, and then are ejected mass-independently for subsequent mass analysis.
U.S. Pat. No. 5,420,425 teaches that ions can be trapped and stored in a two-dimensional RF quadrupole and scanned out mass-dependently, using the technique of mass selective instability. According to that patent the device disclosed therein was conceived in order to improve ion sensitivities, detection limits, and dynamic range, by increasing the volume of the trapping chamber in the axial dimension. The mass selective instability mode of ion ejection (and all other mass analysis scanning modes described in U.S. Pat. No. 5,420,425) involve ejecting ions out of the trapping chamber in a direction orthogonal to the center axis of the device, i.e. radially.
There are several disadvantages of radial ejection of ions from a two-dimensional RF quadrupole. One disadvantage is that radial ejection expels ions through or between the quadrupole (or higher order multipole) rods. This forces the ions through regions of space for which there are significant RF field imperfections. The effect of these imperfections is to eject ions at points not predicted by the normal stability diagram.
Radial ejection from a two-dimensional RF quadrupole has the further disadvantage of providing a poor match between the dimensions of the plug of ejected ions and conventional ion detectors. In a linear or curved rod structure, radially ejected ions will exit throughout the length of the device, i.e. with a rectangular cross-section of length corresponding to the rods themselves. Most conventional ion detectors have relatively small circular acceptance apertures (e.g. less than 2 cm.sup.2) that are not well-suited for elongated ion sources.
Mass selective instability for radial ion ejection of ions from a two-dimensional RF quadrupole has additional problems. Ions ejected radially from such a device will exit with a diverging spacial profile with a characteristic solid angle. Some of the ejected ions will hit the rods and be lost. In addition, radially ejected ions will leave the trapping structure in opposite directions. Multiple ion detectors would be required to collect all of the ions made unstable by this and similar techniques. Ions ejected away from the detector(s) or which encounter one of the electrodes are lost and therefore do not contribute to the measured ion signal. Therefore only a small fraction of trapped ions would normally be collected, despite the very high storage ability of this device.